"Space robot with interchangeable accessories"
Sounds great for me as a kid. Together with a few Shogun Warriors and lots of Airfix soldiers it was great fun to play with them.
These Ark toys had been the Lego-space-monster-Technic of the late 70ties.
But back in the day you didn't find them in every toy store here in germany.
Big Jim, Action Team (GI Joe), Kenner Star Wars etc had been more popular than japanese Diecats Toys. We didn't have any cool manga series here (only Captain Future) like Goldorak etc.

Today loose specimen pop up on german ebay from time to time,
but not very often.
You don't find them on flea-markets or antique stores.
I bought boxed ones on german & belgium ebay over the years.
But boxed ones (complete and mint) are very scarce these days.
I bought a few complete only in styrofoam, no outer box.
Because of the big window the Ark boxes had been very fragile, so many
had been destroyed over the years.